The stipulation will be something like "if Cena loses, he doesn't get another WWE Title shot for as long as Punk is champion".

Or, it sounds far-fetched, but it could be if Cena loses, Punk is allowed to change the belt?

Afterall, the current design is known as being "Cena's belt" as he was the one who introduced it. Punk being this guy who's all about respect and is a "nice guy" all that, would want Cena's permission first, before changing it.